The short-chain halogenated polysilanes in question are often also referred to as halogenated oligosilanes or oligohalosilanes and are compounds which each have at least one direct Si—Si bond and the substituents of which comprise halogen and hydrogen, but preferably consist exclusively of halogen or of halogen and hydrogen.
Long-chain polysilanes have longer chains than the short-chain polysilanes defined above, but otherwise have the same structure. More particularly, long-chain polysilanes are also composed of directly interconnected silicon atoms, the free valencies of which are saturated by chlorine atoms.
Various methods of decomposing halogenated silanes are known. They are predominantly used to produce elemental silicon. DE 1102117 B or U.S. Pat. No. 3,042,494 A, for instance, disclose thermal decomposition of gaseous silicon compounds at heated silicon rods or silicon wires to deposit silicon. Temperatures of 800-1300° C. and HSiCl3 or SiCl4 in a mixture with hydrogen are frequently employed. EP 0282037 A2 describes thermal decomposition of gaseous chlorinated polysilanes SinCl2n+2 (n=2-4) on silicon and at least 700° C. to deposit polycrystalline silicon. GB 838378, for example, discloses formation of elemental silicon powder by applying an electric discharge to gaseous mixtures of hydrogen and halogenated monosilanes. EP 0264722 A2 describes thermal decomposition of polysilanes SinCl2n+2 (n≧2) at 250-700° C. to produce amorphous silicon. U.S. Pat. No. 4,292,342, for example, describes deposition of silicon on a preheated substrate after a silicon compound has been reduced with hydrogen in a plasma flame. US 2004/0250764 A1 discloses deposition of silicon from the reaction of SiF4 with hydrogen in a plasma zone onto silicon particles which descend through the plasma zone in a rotating cylinder. U.S. Pat. No. 6,451,277 B1, WO 02/40400 A1 or JP 01197309 A describe deposition of silicon by thermal reaction of gaseous halogenated silanes with hydrogen onto silicon seed grains in fluidized bed reactors.
Various heating methods are disclosed such as conventional resistance heating, microwave irradiation of reactor contents or preheating the reaction gases and the seed grains outside the reaction zone. Gaseous sources of silicon employed include beside halogenated monosilanes also compounds SinCl2n+2 (n=2-4). U.S. Pat. Nos. 4,070,444 or 4,138,509, for example, describe that solid (SiF2)x obtained from gaseous SiF2, or other perfluorinated polysilanes release elemental silicon on heating.
O. H. Giraldo, W. S. Willis, M. Marquez, S. L. Suib, Y. Hayashi, H. Matsumoto, Chemistry of Materials 10 (1998) 366-371 disclose that deposition of amorphous silicon from the gas phase leads to products containing major amounts of other elements. An electric glow discharge in a SiCl4/H2/He gas mixture under atmospheric pressure produces amorphous silicon which in addition to hydrogen still contains about 1% Cl.
U.S. Pat. No. 4,374,182 discloses that perchlorinated polysilanes having a molecular weight greater than that of Si2Cl6 can be decomposed at temperatures between 500° C. and 1450° C. in an inert atmosphere or in vacuo to form silicon. JP 01192716 A describes reduction of perchlorinated polysilanes SinCl2n+2 at temperatures of 250-1300° C. to produce amorphous or crystalline silicon.
EP 140660 B1 discloses that polymeric halosilane layers are deposed on substrates from gaseous halogenated disilanes or polysilanes at 250-550° C.
R. Schwarz, Angewandte Chemie 51 (1938) 328, R. Schwarz, U. Gregor, Zeitschrift für anorganische und allgemeine Chemie 241 (1939) 395, R. Schwarz, A. Köster, Zeitschrift für anorganische und allgemeine Chemie 270 (1952) 2, M. Schmeisser, M. Schwarzmann, Zeitschrift für Naturforschung 11 b (1956) 278, M. Schmeisser, P. Voss, Zeitschrift für anorganische und allgemeine Chemie 334 (1964) 50, P. W. Schenk, H. Bloching, Zeitschrift für anorganische und allgemeine Chemie 334 (1964) 57 report that tetrahalosilanes react with elemental silicon at high temperatures to form perhalogenated polysilanes (SiX2)x. R. Schwarz, H. Meckbach, Zeitschrift für anorganische und allgemeine Chemie 232 (1937) 241, R. Schwarz, Angewandte Chemie 51 (1938) 328, R. Schwarz, U. Gregor, Zeitschrift für anorganische und allgemeine Chemie 241 (1939) 395, R. Schwarz, A. Köster, Zeitschrift für anorganische und allgemeine Chemie 270 (1952) 2 or M. Schmeisser, P. Voss, Zeitschrift für anorganische und allgemeine Chemie 334 (1964) 50 disclose that the thermal treatment of chlorinated polysilanes or else of nonvolatile perchlorinated oligosilanes in a vessel leads to products having compositions SiClx (x<2) depending on the temperature and the treatment time.
GB 702,349 discloses that the reaction of silicon alloys with chlorine gas at 190-250° C. leads to a mixture of perchlorinated polysilanes (PCS) being condensed out of the gas stream. The average molar mass of these mixtures is relatively low, since distillation reveals that only 2% of the silanes have n larger than 6.
DE 31 26 240 C2 describes wet-chemical production of PCS from Si2Cl6 by reaction with a catalyst. The mixtures obtained still contain the catalyst and are therefore washed with organic solvents. The PCS thus obtained are highly branched. Further wet-chemical methods are presented, for example, in E. Hengge, D. Kovar, Journal of organometallic Chemistry 125 (1977) C29 or in US 2007/0078252 A1:                reduce halogenated aryloligosilanes with sodium or lithium and then use HCl/AlCl3 to detach aromatics;        transition metal catalyzed dehydrogenative polymerization of arylated H-silanes and subsequent dearylation with HCl/AlCl3;        anionically catalyzed ring opening polymerization (ROP) of (SiCl2)5 with tetrabutylammonium fluoride (TBAF);        ROP of (SiAr2)5 with TBAF or Ph3SiK and subsequent dearylation with HCl/AlCl3.        
By combining dearylation, ring opening and hydrogenation of halosilanes in a suitable manner it is possible to obtain partially halogenated short-chain polysilanes as described, for example, in E. Hengge, G. Miklau, Zeitschrift für anorganische and allgemeine Chemie 508 (1984) 33.
In addition to thermal decomposition, it is the reaction between Cl2 and silicon, yielding mainly SiCl4, which is also used for producing chlorinated polysilanes. To enhance the yield of perchlorinated polysilanes, the reaction temperature has to be lowered. EP 283905 produces mixtures of Si2Cl6 and Si3Cl8 with SiCl4 by copper-catalyzed reaction of Si at 140-300° C. The yield of polysilanes reaches more than 40% based on the amount of Si used. SiCl4 is present in the product mixture at less than 50% by weight only.
It is further known to produce halogenated polysilanes of this type via a plasma-chemical method. DE 10 2005 024 041 A1 relates to a method for producing silicon from halosilanes which comprises a first step of converting the halosilane to a halogenated polysilane by producing a plasma discharge and a subsequent, second step of decomposing the halogenated polysilane to silicon by heating to more than 500° C. The polysilane is obtained in the form of a waxily white to yellow-brown or brown solid material of little compactness.
WO 81/03168 further describes a high-pressure plasma process for synthesis of HSiCl3 while PCS are obtained as minor by-products. Since these PCS are generated at extremely high gas temperatures, they are relatively short-chained and highly branched. In addition, this PCS has a high hydrogen content due to the hydrogenative conditions (HSiCl3 synthesis). DE 10 2006 034 061 A1 additionally describes a similar reaction in which gaseous and liquid PCS are obtained with Si2Cl6 as a main component (page 3, [00161]).
The predominant proportion of prior art methods of decomposing halogenated silanes produce elemental silicon. The degree of decomposition of the halogenated silanes used and, hence, the composition of products is oftentimes very difficult to control. The residual concentrations of halogen and hydrogen are low. Solid materials having comparatively low silicon contents are exclusively by-produced. The likewise predominant proportion of prior art methods rely on gaseous halogenated silanes being fed into the decomposition process and therefore cannot be used for decomposing high-boiling compounds. Monosilanes are predominantly used as starting compounds, the decomposition of which can make it necessary to feed a further reaction gas such as hydrogen.
The known methods are generally also costly and inconvenient in that short-chain halogenated polysilanes are only formed as by-products and therefore are not obtainable in this way on an industrial scale.
It could therefore be helpful to provide a method for producing short-chain halo-genated polysilanes and/or short-chain halogenated polysilanes and silicon, the method enabling the production of such products in a simple and inexpensive manner on an industrial scale.